Among light emitting elements, LED (Light Emitting Diode) elements have been put into practical use as light emitting elements with high luminance and low power consumption.
For example, a LED that is a semiconductor light emitting element formed using a nitride semiconductor (hereinafter exemplarily indicated by “GaN”) is expected to be used as a light source for illumination, replacing a fluorescent light. The development of such LED has been advanced with the aim of increasing lumen/watt (i.e., luminous efficacy) in terms of performance, and reducing the cost/lumen in terms of cost. Light that is emitted through recombination of holes and electrons in the active layer is released to the air from GaN. However, since the refractive index of GaN is as high as about 2.5 as compared to the refractive index of the air that is 1.0, approximately a little less than 70% of light would be confined in the semiconductor due to total reflection at the interface between GaN and the air, and finally, the light would change to heat and thus disappear. Thus, finding how to extract light from GaN to the outside is a significant object to be achieved to improve the performance and reduce the cost.
Meanwhile, high-intensity LEDs that emit deep ultraviolet rays with a wavelength of 220 to 350 nm are expected to be applied to a wide range of fields such as sterilization/water purification, various medical fields, high-density optical recording, high-color-rendering LED illumination, and a high-speed degradation process for pollutants. However, the external quantum efficiency of deep ultraviolet LEDs so far has been about several % at the highest, which is significantly low even when it is compared with the external quantum efficiency of blue LEDs that is several ten %. Thus, the deep ultraviolet LEDs have been difficult to be put into practical use.
The external quantum efficiency (EQE) of a LED is determined by the product of the internal quantum efficiency (IQE), the electron injection efficiency (EIE), and the light extraction efficiency (LEE) (i.e., represented by the equation: EQE=IQE×EIE×LEE). Thus, an improvement in light extraction efficiency, as well as improvements in internal quantum efficiency and electron injection efficiency, is a factor that significantly contributes to an improvement in external quantum efficiency.
For example, a deep ultraviolet LED element shown in FIG. 1A is formed by sequentially forming an n-type AlGaN layer 5/an AlN buffer layer 3, an active layer including AlGaN/GaN multiple quantum wells (hereinafter exemplarily indicated by an AlGaN active layer) 7, a p-type AlGaN layer 9, and a Ni/Au electrode layer 11 on a sapphire substrate 1. The n-type AlGaN layer 5 has an n-type electrode 4 formed thereon.
Light that is emitted through recombination of holes and electrons in the AlGaN active layer 7 passes through the sapphire substrate 1 as indicated by arrows L1 to L3, and is then released to the air form a rear surface 1a (i.e., a light extraction plane) thereof. Herein, in comparison with the refractive index of the air that is 1.0, the refractive index of sapphire is as high as 1.82. Provided that the incident angle is θi, the critical angle θc at the interface between the rear surface 1a of the sapphire substrate and the air is computed as 33.3° from Thus, light that has entered at an angle above the critical angle θc would be confined within the nitride semiconductor layers 3, 5, and 7 and the sapphire substrate 1 due to total reflection, and would finally change to heat and disappear (L2 and L3). The percentage of light that would disappear through such heat dissipation is as high as 70 to 90%. Thus, finding how to extract light to the outside that would otherwise disappear inside is an object to be achieved to improve the performance.
Meanwhile, a blue LED shown in FIG. 1B is formed by, for example, sequentially forming an n-type electrode 24, an n-type GaN layer 23, an active layer 25 containing GaN or the like, a p-type GaN layer 27, an ITO transparent electrode layer 29, and a SiO2 protective film 31 on a sapphire substrate 21. Light that is emitted from the GaN active layer 25 is output in the upward and downward directions of the SiO2 protective film, but more than 50% of light emitted from the top protective film (the refractive index of which is 1.46 if it contains SiO2, for example) to the air would be totally reflected at an angle above the critical angle, and thus disappear inside. Likewise, there is also a problem in that, at the interface between the bottom n-type GaN layer 23 (which has a refractive index of 2.50) and sapphire substrate 21 (which has a refractive index of 1.78 at a wavelength of 455 nm), approximately a little less than 50% of light emitted from the n-type GaN layer 23 to the sapphire substrate 21 would be totally reflected and thus disappear inside.
In order to solve such problems, for example, in Patent Literature 1 below, ingenuity is exercised by forming holes, which are open in one of semiconductor layers including a p-type nitride semiconductor layer and an active layer in the stacked direction thereof, as a photonic crystal periodic structure with a photonic band gap, and extracting light from above and below the semiconductor layers in the stacked direction thereof while blocking light that propagates through a waveguide that is parallel with the semiconductor layers.
In addition, in Patent Literature 2, ingenuity is exercised by forming a photonic crystal periodic structure, which has periods that are set to ¼ to 4 times the wavelength of light emitted from an active layer, on the rear surface of a sapphire substrate, and extracting light to the air from the rear surface of the sapphire substrate while controlling total reflection.
In Patent Literature 3, ingenuity is exercised by forming a photonic crystal periodic structure with a photonic band as holes in an active layer, and extracting light from above and below the active layer while blocking light that propagates through a waveguide that is parallel with the active layer and semiconductor layers above and below it.
In Patent Literature 4, ingenuity is exercised by forming, after creating a predetermined LED structure, a photonic crystal periodic structure with a photonic band in an n-type semiconductor layer from which a sapphire substrate has been removed, and extracting light from the n-type semiconductor layer.
Further, in Patent Literature 5, a protrusion periodic structure (i.e., a moth-eye structure) that is less than or equal to ⅓ of the light emission wavelength is formed at the interface between a sapphire substrate and a nitride semiconductor layer, so that total reflection at the interface is suppressed, and light is extracted from the rear surface of the substrate.
In Patent Literature 6, ingenuity is exercised by forming holes, which penetrate through an ITO transparent electrode, a p-type semiconductor layer, an active layer, and an n-type semiconductor layer, as a photonic crystal periodic structure with a photonic band gap, and extracting light from above and below the layers in a direction perpendicular to the layers while blocking light that propagates through a waveguide that is parallel with the layers.
Further, in Non Patent Literature 1, ingenuity is exercised by forming a periodic structure with a moth-eye structure on the rear surface of a sapphire substrate, and extracting light while suppressing total reflection from the rear surface of the sapphire substrate.